This invention relates to a display device, and in particular to a display device, which is suitable for driving an active matrix liquid crystal display.
According to a prior art method for driving an active matrix liquid display, as described in a report in the meeting materials of the National Meeting of the Television Society of Japan, 1983, P. 121-122, rectangular pulses having a constant pulse width are applied as a scanning voltage to the scanning wiring (horizontal side wiring) for driving electrodes of thin film transistors (hereinbelow abbreviated to TFT elements) in the display portion, while retarding them successively for every line. On the other hand the signal voltage corresponding to the display information in the display portion is applied to the signal wiring (vertical side wiring) driving the gate electrode of the TFT elements in synchronism with the timing of the scanning voltage applied to the scanning wiring.
The timing to apply the scanning voltage and the signal voltage to the TFT elements is different by the line at a time scanning method and by the point at a time scanning method. However, by either of the methods, it is assumed that the rise time t.sub.r and the fall time t.sub.f of the scanning voltage applied to the gate electrode of the TFT elements in the display portion are sufficiently short and that distortions in the waveform are negligible.
However, specifically in the case where a material having a large resistivity is used for the scanning wiring, and in the case where the area of the display portion is increased and the wiring is elongated, both the rise time t.sub.r and the fall time t.sub.f of the scanning voltage are elongated on the side distant from the input terminal of the scanning voltage, what gives rise to distortions in the waveform. Consequently, these distortions in the waveform in the scanning wiring become larger for the pixels with increasing distance from the input terminal of the scanning voltage.
These distortions in the waveform give rise to phenomena that the voltage applied to the gate electrode of the TFT elements is different for every pixel and phenomena that the timing of the pulse for the scanning voltage is shifted from that for the signal voltage. In this way they can be a cause of deterioration of display quality due to inhomogeneity of the display state or a cause of error of display information.
With respect to the points discribed above, in a prior art display device, no attention has been paid to these distortions in the waveform and the timing shift and thus problems such as lowering of the display quality, errors in the display information, etc. have been provoked.